Temperature dependence of the upper critical field in the model of a superconductor with singular points near the Fermi surface

We consider a generalization of BCS theory to the case where the energy spectrum of parent charge carriers near the Fermi energy has a nonlinear character. This nonlinearity can cause an abrupt decrease in the coherence length, growth of the density of states and transition temperature, and non-analyticity of the vertex part as a function of the momentum. As the density of states increases the model exhibits a tendency toward negative curvature of the critical field at the transition point (although a sign change of the curvature is possible near it). Positive curvature can show up upon depletion of the density of states, which in fact corresponds to a departure of the crystal parameters from the conditions maximizing the transition temperature. Fiz. Tverd. Tela (St. Petersburg) 41, 975–978 (June 1999)     

Enhancement of fluctuation effects in superconductors with singularities near the Fermi surface. Paraconductivity

It is shown that the existence of certain topological features on the equipotential surfaces of seed charge carriers near the Fermi level in a superconductor can have an effect on the character of the temperature anomalies in the fluctuation corrections. An expression is obtained for the fluctuation correction to the conductivity in a superconductor model, in which high values of the transition temperature are caused by an increase in the density of states of seed charge carriers in the region near the Fermi surface, where the interaction constant is nonzero. An anomaly in the density of states leads to nonanalyticity of the frequency dependence of the Cooper-pair propagator and to alteration of the fluctuation relaxation time relative to the classical value in the BCS theory. This situation is responsible for enhancement of the role of fluctuations in raising the power of the singularity in the temperature corrections. Possible interpretations of the experimental data available in the literature are discussed from this point of view. Fiz. Tverd. Tela (St. Petersburg) 40, 603–605 (April 1998)     

Fermi hole near a metal surface

The canonical density matrix for a non-interacting electron gas bounded by a planar infinite barrier is calculated exactly by solving the Bloch equation for free particles. A closed expression describing the shape and size of the Fermi hole is thereby obtained.     

The Fermi surface of a superconductor: OsB2

Osmium diboride has been known for some time as a low compressibility material and a superhard material. It is suitable for hard coating applications. It is also a superconductor below 2.1 K. Using first‐principles calculations, the author investigated the geometry of its Fermi surface (FS) and calculated the related physical quantities. The theoretical results are used to predict the frequencies of the Shubnikov–de Haas quantum oscillations. Comparison with recent measurements of the magneto‐resistance oscillations in osmium diboride is made.

     

About coupling constant singularities in quantum electrodynamics

The contribution of the multibubble insertions in the photon propagator to the anomalous magnetic moment of the electron is reexamined. Using the asymptotic form of the photon propagator one finds a softer singularity than that found in a recent analysis.     

Fermi surface of the 2D Hubbard model at weak coupling

We calculate the interaction-induced deformation of the Fermi surface in the two-dimensional Hubbard model within second order perturbation theory. Close to half-filling, interactions enhance anisotropies of the Fermi surface, but they never modify the topology of the Fermi surface in the weak coupling regime.     

Evidence for a striped phase of high T c superconductor at the Fermi surface

A complete Fermi surface in the normal state of superconducting Bi₂Sr₂CaCu₂O_8+δ at the optimum doping (T _c = 91K) has been measured by angle scanning photoemission spectroscopy using synchrotron radiation. The Fermi surface reveals broken segments and hot spots of high photointensity along lines of k _y = n:0:23Å^?1 in the (π; π) direction. The pseudogap observed at (π,0) in underdoped materials is one of this missing segments. The results indicate the presence of characteristic subbands of a superlattice of quantum stripes. This is formed by an ordered array of ～ 14Å large stripes.     

Unraveling the symmetry of the hole states near the Fermi level in the MgB2 superconductor

We use x-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS) to study the fine structure at the K edge of boron in MgB(2). We observe in XAS a peak of width 0.7 eV at the edge threshold, signaling a narrow energy region with empty boron p states near the Fermi level. The changes in the near edge structure observed in EELS with direction of the momentum transfer imply that these states have p(x)p(y) symmetry. Our observations are consistent with electronic structure calculations indicating a narrow energy window of empty p(x)p(y) states that falls to zero at 0.8 eV above the Fermi level. The disappearance of the p(x)p(y) feature in EELS at grain boundaries suggests that this signature may become powerful in probing superconductivity at nanoscale.     

Time behaviour of order parameter perturbations near the surface of a superconductor

The time-dependent Ginzburg-Landau (GL) equations have been used for calculating some problems associated with the possibility of creation and penetration of flux lines near the surface of superconductors. Using the solutions of the static GL equations and modifying the time-dependent GL equation for the order parameter by simple coordinate transformations, the time behaviour of special forms of order parameter perturbations (pointlike, linear, laminar) is obtained in analytical form, suitable for further use in the iteration procedure, outlined also in this paper. All the treated perturbation forms are suitable for obtaining analytical solutions, at least for small times after the perturbation. The correctness of the used approximations is verified for the laminar perturbation. Some preliminary results for this form of perturbations, as well as for linear perturbations parallel to the vector potential, are also given.     

Doping induced evolution of Fermi surface in low carrier superconductor Tl-doped PbTe

We have performed high-resolution angle-resolved photoemission spectroscopy on Tl-doped PbTe. We observed a distinct energy shift of the valence band and core levels upon Tl doping, together with the evolution of a small hole pocket around the X[over] point in the Brillouin zone, while no clear evidence for the localized states near the Fermi level is observed. These experimental results suggest that direct hole doping into the valence band and resultant emergence of a small Fermi surface are responsible for the metallic conductivity in Tl-doped PbTe.     

Quark-pion coupling constant in a chiral quark model

Incorporating chiral-symmetry to the potential model of quarks with confining potentialU(r)=1/2 (1 +γ°)ar ² with m _q =10 MeV anda=2.273 fm⁻³ that gives a reasonable quark-core contribution to μ _p , 〈r ² 〉 _p ^1/2 andg _A , the quark-pion coupling constant for quarks in a nucleon is estimated.G _qqπ ² /4π obtained between 0.4 and 0.5 is consistent with those extracted from experimental vector meson decay-width ratios by Suzuki and Bhaduri. The nucleon-pion coupling constantG _NNπ ² /4π comes out to be of the order of 13.1 in reasonable agreement with the experimental value.     

Itinerant ferromagnetism entrenched by the anisotropy of spin−orbit coupling in a dipolar Fermi gas

We investigate the itinerant ferromagnetism in a dipolar Fermi atomic system with the anisotropic spin−orbit coupling (SOC), which is traditionally explored with isotropic contact interaction. We first study the ferromagnetism transition boundaries and the properties of the ground states through the density and spin-flip distribution in momentum space, and we find that both the anisotropy and the magnitude of the SOC play an important role in this process. We propose a helpful scheme and a quantum control method which can be applied to conquering the difficulties of previous experimental observation of itinerant ferromagnetism. Our further study reveals that exotic Fermi surfaces and an abnormal phase region can exist in this system by controlling the anisotropy of SOC, which can provide constructive suggestions for the research and the application of a dipolar Fermi gas. Furthermore, we also calculate the ferromagnetism transition temperature and novel distributions in momentum space at finite temperature beyond the ground states from the perspective of experiment.     

Breakup of the Fermi surface near the mott transition in low-dimensional systems

We investigate the Mott transition in weakly coupled one-dimensional (1D) fermionic chains. Using a generalization of dynamical mean field theory, we show that the Mott gap is suppressed at some critical hopping t{ perpendicular}{c2}. The transition from the 1D insulator to a 2D metal proceeds through an intermediate phase where the Fermi surface is broken into electron and hole pockets. The quasiparticle spectral weight is strongly anisotropic along the Fermi surface, both in the intermediate and metallic phases. We argue that such pockets would look like "arcs" in photoemission experiments.     

The local autocorrelation time near the surface of a system with uniaxial anisotropy in a transverse field

A three-dimensional semi-infinite system with strong uniaxial anisotropy ina transverse field is considered. The behaviour of the local autocorrelation time for the component of the order parameter in the direction parallel to the easy axis near the second-order phase transition for this component induced by the transverse field is given. The effect of the surface on this behaviour is discussed. The Landau approximation is used.     

Electrostatic energy and screened charge interaction near the surface of metals with different Fermi surface shape

Using the Poisson equation Green function for a self-consistent field in a spatially inhomogeneous system, expressions for the electrostatic energy and screened charge interaction near the surface of a semi-infinite metal and a thin quantizing film are derived. It is shown that the decrease law and Friedel oscillation amplitude of adsorbed atom indirect interaction are determined by the electron spectrum character and the Fermi surface shape. The results obtained enable us to explain, in particular, the submonolayer adsorbed film structure on the W and Mo surfaces.     

Local enhancement of the effective mass near the Fermi surface in nuclear matter

It is shown that the effective mass of nucleons in nuclear matter has a pronounced and narrow maximum near the Fermi surface, in contradistinction with previous belief that this local enhancement exists only in finite nuclei. Implications of this finding for self-consistent Hartree-Fock calculations of finite nuclei are pointed out.     

Odderon with a running coupling constant

The running coupling is introduced into the equation for the odderon via the bootstrap relation. It is shown that the previously found odderon state with a maximal intercept, which is constructed from an antisymmetric pomeron wave function, continues to exist in the running coupling case. Its intercept is found to remain equal to unity independent of the behaviour assumed for the running coupling at low momenta.